Bring condensed milk and butter to a boil and cook for about 6-10 min, stirring constantly (be careful to not let it burn because it will caramelize and become brown). The mixture should be slightly thickened. Add whipping cream, stir and remove from the heat.
Divide the mixture into two portions. Add the hazelnut spread to one and white chocolate to the other, stir well until you get a homogenous mixture. Check the consistency, but take into consideration that the filling will become a bit firmer as it cools down. If you would like your filling to be more liquid, add more whipping cream, or add some milk in powder, if you want it less liquid.
(Optional) To make the balls that you see, take some of the filling and mix in some powdered milk to make it firm just enough that you can roll some balls and then cover them with powdered milk. Take half of the chocolate egg and fill the half of it with white filling. Take some wafers, roughly crush them and place on the white filling. Then fill with chocolate filling almost to the edges of the egg.
Decorate with wafers, crunchy pearls, the balls you made and with whatever you want. Sprinkle with milk in powder and drizzle some more filling on top. Cut into it and watch how it oozes out mmm...